This proposal examines how a growth factor signaling pathway is linked to synapse structure and function. Growth factors are potent neuromodulators and play diverse roles at synapses. They regulate fundamental features of synaptic biology including baseline transmitter release, synapse morphology, and plasticity. Given their importance directing synapse organization, it is important to define the molecular and cellular mechanisms coupling synaptic growth factor signaling to synapses and neuronal activity. BMP/TGF? family members are evolutionarily conserved regulators of synapse structure and function. In particular, a BMP/TGF? pathway has neurotrophic pathway activity at the Drosophila NMJ. BMP pathway mutants display striking defects in NMJ morphology and function. Remarkably, distinct ligand pools independently regulate these synaptic features. We demonstrate that the presynaptic pool regulates synaptic structure and function, while the postsynaptic pool directs overall growth of the NMJ terminal. Understanding how these information channels are separated at an endogenous synapse is essential to understand how distinct synaptic features are independently controlled. Our entry point to this work is the novel neuronal transmembrane protein Crimpy. We have demonstrated that Crimpy enables discrimination between pre- and postsynaptic BMP pools. Crimpy binds a BMP homolog called Gbb and traffics it to presynaptic dense core vesicles (DCVs). Without Crimpy, Gbb is no longer found in DCVs and is not released by presynaptic activity. In the absence of Crimpy, pre- and postsynaptic ligand pools cannot be distinguished, and the NMJs are characterized by aberrant trophic signaling at the expense of the presynaptic synapse-organizing cue. In this proposal, we build on our novel preliminary findings to define the role of activity-induced presynaptic BMP signaling in synapse structure and function. First, we define the molecular identity of the pre- and postsynaptic signals. Because Crimpy is key for marking the presynaptic pool, we will define biochemically the role of Crimpy in BMP signal transduction. Second, we will elucidate how activity-dependent BMP signals direct synapse organization. We test the hypothesis that the Crimpy-mediated presynaptic Gbb signal is a local and acute cue instructive cue driving synapse organization. And third, we will define the downstream signaling cascade. We have exciting preliminary evidence that a novel BMP receptor transduces the activity-dependent signal. We will establish the receptor's role in the pathway and characterize novel downstream components of a non-canonical activity-dependent BMP cascade.